This application claims the benefit of Japanese patent application No. 10-283399, filed Sep. 18, 1998, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an optical pickup system for recording to or reading recorded information from an optical disc, and more particularly, to an optical pickup system for use with both a DVD (Digital Versatile Disc) and a CD (Compact Disc).
2. Description of the Related Art
A DVD is an optical disc that is capable of recording a large amount of digital information, such as motion picture and computer information. A DVD""s recording scheme allows the recording of digital information on a disc having the same diameter as a CD, but at a recording density of six to eight times higher than the recording density of a CD. In particular, the diameter of a DVD and a CD is 12 cm.
Various methods and devices have been developed for a DVD to achieve such high density recording. For example, the wavelength of a laser light source used for a DVD is set to 650 or 634 nm (nanometers), which is much shorter than a wavelength of 780 nm used for a CD. Moreover, the numerical aperture of an objective lens for a DVD is set to 0.6, which is larger than a value of 0.45 of an objective lens for a CD. Furthermore, MPEG is used as the data compression algorithm. As a result, a DVD has a density of about 5 gigabytes, which is about eight times the density of a CD, on one side of the disc.
Due to such high density recording on a DVD, the spot diameter of the laser beam, which reads pit information from a DVD, is set to a value fairly smaller than that of a laser beam used to read information from a CD. The spot diameter of the laser beam is proportional to the wavelength xcex of the laser being used and is inversely proportional to the numerical aperture of the objective lens. Accordingly, a laser beam can have a small spot diameter if the wavelength of the laser light source is short and the numerical aperture of the objective lens is large.
Moreover, it is desired that a DVD player reproduce a CD in addition to a DVD since a DVD and a CD use a similar recording format. For example, the information items are digitally stored in a spiral track in both a DVD and a CD. However, some differences between a DVD and a CD cause problems. For example, one major difference is disc thickness. The disc substrate thickness of a DVD is equal to 0.6 mm, while that of a CD is equal to 1.2 mm. In particular, in an optical system for a DVD in which the spot diameter of a laser beam is small, the spot diameter of the laser beam becomes enlarged when using a CD due to a difference in aberration caused by the disc substrate thickness. Therefore, the information on the CD cannot be read by the same optical pickup system that is being used for a DVD.
To solve the problem caused by the disc size, a conventional optical pickup system has a liquid crystal panel that controls the numerical aperture. Such an optical pickup system with a liquid crystal panel is disclosed in Japanese laid open patent publication No. HEI 9-106566. In such a system, the liquid crystal panel has a pair of transparent electrodes with a certain aperture pattern. Furthermore, the liquid crystal panel gives a phase difference of xc2xc-wavelength, or an odd multiple of this wavelength, to the passing light. A voltage applied to the transparent electrodes of the panel is controlled to limit the numerical aperture. As a result, the entire liquid crystal panel functions as a xc2xc-wavelength plate during DVD reproduction and only the aperture area on the transparent electrodes functions as a xc2xc-wavelength plate during CD reproduction.
In a conventional optical system, such as the one described in the foregoing description, a laser beam from a laser source is guided to a recorded surface of a disc through a polarized beam splitter and a liquid crystal panel that functions as a xc2xc-wavelength plate. This beam is reflected from the disc and passes through the polarized beam splitter and the liquid crystal panel. The reflected beam is polarized 90-degrees with respect to the incident laser beam as a result of passing through the liquid crystal panel twice. Then, the reflected beam is guided to a photo detector, which is positioned in a different direction from the light source, by the polarized beam splitter. However, some optical discs have birefringence, which may also cause polarization of the reflected beam. So, the reflected beam is polarized due to the birefringence and the liquid crystal panel. As a result, the reflected beam is elliptically polarized due to the birefringence rather than being linearly polarized by the liquid crystal panel. In other words, the reflected beam contains vertical as well as parallel components in the polarization angle of the laser beam.
As a result of the polarization due to the birefringence, the amount of light received by the photo detector decreases due to the phase differences. For example, when the phase difference caused by the birefringence is xcfx80 radian, no beam is guided to the photo detector, and the information recorded on the optical disc cannot be read. Also, the amount of the birefringence depends on the area on the optical disc and thus, the polarization of the reflected beam is changed in accordance with disc rotation. As a result, jitters in the output signal of the photo detector increase since the amount of the reflected beam changes upon disc rotation. Moreover, a system is needed to eliminate the effects of aberration caused by the thickness difference in addition to eliminating the influence of birefringence.
Accordingly, the present invention is directed to an optical pickup system that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide an optical pickup system with a liquid crystal panel that can reproduce a DVD and a CD by substantially reducing an influence of the birefringence of the optical disc.
Additional features and advantages of the invention will be set forth in the description which follows, and will be apparent from the description, or may be learned by practice of the invention. The objects and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages in accordance with the purpose of the invention, as embodied and broadly described herein, an optical pickup system includes a light source; an objective lens; a beam splitter for splitting light received from the light source and reflected light from an optical disc; a detector for receiving the reflected light; a liquid crystal panel for providing passing light with a phase difference in accordance with a control signal; and a control means for providing the control signal to the liquid crystal panel to eliminate an influence of birefringence of the optical disc.
In another aspect, the present invention provides an optical pickup system for reproducing information from a plurality of optical discs including a first optical disc of a first thickness and a second optical disc of a second thickness that includes a light source; an objective lens; a beam splitter for splitting light received from the light source and reflected light from an optical disc; a detector for receiving the reflected light; a liquid crystal panel for providing passing light with a phase difference in accordance with a control signal; and a control means for providing the control signal to the liquid crystal panel, wherein the control means controls the liquid crystal panel such that an influence of birefringence of the first optical disc is eliminated while the optical pickup system reproduces information from the first optical disc, and an influence of birefringence of the second optical disc is eliminated and a numerical aperture is limited while the optical pickup system reproduces information from the second optical disc.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.